| 摘要 |
<p>PURPOSE: To accurately diagnose leakage of a secondary air introducing passage by detecting the existence of secondary air in exhaust air on the basis of a detecting signal from an air-fuel ratio detecting means when a cutoff condition and a supply condition overlap with each other for a prescribed time, and judging it as leakage of an opening-closing means when the secondary air is detected. CONSTITUTION: An overlap of a cutoff condition by an air cut valve 22 generated at supply stopping time of secondary air and a supply condition by an air pump 21 is detected by a judging part 32b. A cutoff condition where the air cut valve 22 cuts off a secondary air introducing passage 20 by a condition overlapping part 32a and a supply condition where the air pump 21 delivers and supplies secondary air in the secondary air introducing passage 20, are overlapped with each other for only a prescribed short time. The existence of the secondary air is detected in this overlapped condition on the basis of the air-fuel ratio in exhaust gas detected by a first air-fuel ratio sensor 16 by a judging part 32b. Therefore, when the secondary air is detected, since it is judged as leakage of the air cut valve 22, valve opening failure can be accurately diagnosed.</p> |
